Separator, method for producing the same and electrochemical device including the same

ABSTRACT

A separator includes a porous substrate, a porous organic-inorganic coating layer formed on at least one surface of the porous substrate, and an organic coating layer formed on the surface of the organic-inorganic coating layer. The porous organic-inorganic coating layer includes a mixture of inorganic particles and a first binder polymer. The first binder polymer contains a copolymer including (a) a first monomer unit including either at least one amine group or at least one amide group or both in the side chain thereof and (b) a (meth)acrylate having a C 1 -C 14  alkyl group as a second monomer unit. The organic coating layer is formed by dispersing a second binder polymer on the surface of the organic-inorganic coating layer, leaving scattered uncoated areas.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of International ApplicationNo. PCT/KR2012/001099 filed on Feb. 14, 2012, which claims priority toKorean Patent Application No. 10-2011-0013312 filed on Feb. 15, 2011,and Korean Patent Application No. 10-2012-0013889 filed on Feb. 10, 2012in the Republic of Korea, the disclosures of which are incorporatedherein by reference.

TECHNICAL FIELD

The present disclosure relates to a separator for an electrochemicaldevice such as a lithium secondary battery, a method for producing theseparator, and an electrochemical device including the separator. Morespecifically, the present disclosure relates to a separator including aporous organic-inorganic coating layer composed of a mixture ofinorganic particles and a binder polymer on a porous substrate, and anelectrochemical device including the separator.

BACKGROUND ART

Recently, there has been growing interest in energy storagetechnologies. As the application fields of energy storage technologieshave been extended to mobile phones, camcorders, notebook computers andeven electric cars, efforts have increasingly been made towards theresearch and development of electrochemical devices. In this aspect,electrochemical devices have attracted the most attention. Thedevelopment of secondary batteries capable of repeatedly charging anddischarging has been the focus of particular interest. In recent years,extensive research and development has been conducted to design newelectrodes and batteries for the purpose of improving capacity densityand specific energy of the batteries.

Many secondary batteries are currently available. Lithium secondarybatteries developed in the early 1990's have received a great deal ofattention due to their advantages of higher operating voltages and muchhigher energy densities than conventional batteries using aqueouselectrolyte solutions, such as Ni—MH batteries, Ni—Cd batteries andH₂SO₄—Pb batteries. However, such lithium ion batteries suffer fromsafety problems, such as fire or explosion, encountered with the use oforganic electrolytes and are disadvantageously complicated to fabricate.In attempts to overcome the disadvantages of lithium ion batteries,lithium ion polymer batteries have been developed as next-generationbatteries. However, additional research is still urgently needed toimprove the relatively low capacities and insufficient low-temperaturedischarge capacities of lithium ion polymer batteries in comparison withlithium ion batteries.

Many companies have produced a variety of electrochemical devices withdifferent safety characteristics. It is very important to evaluate andensure the safety of such electrochemical devices. The most importantconsideration for safety is that operational failure or malfunction ofelectrochemical devices should not cause injury to users. For thispurpose, safety regulations strictly prohibit the dangers (such as fireand smoke) of electrochemical devices. In connection with the safetycharacteristics of a lithium secondary battery including a separator,overheating of the lithium secondary battery may cause thermal runawayor puncture of the separator may pose an increased risk of explosion. Inparticular, a porous polyolefin substrate commonly used as a separatorof a lithium secondary battery undergoes extreme thermal shrinkage at atemperature of 100° C. or higher due to its material characteristics andproduction processes including elongation. This thermal shrinkagebehavior may cause short circuits between a cathode and an anode.

Various proposals have been made to solve the above safety problems ofelectrochemical devices. For example, Korean Unexamined PatentPublication No. 10-2007-231 discloses a separator which includes aporous organic-inorganic coating layer formed by coating a mixture ofinorganic particles and a binder polymer on at least one surface of aporous substrate. The inorganic particles present in the porousorganic-inorganic coating layer coated on the porous substrate serve asspacers that can maintain a physical shape of the porousorganic-inorganic coating layer to inhibit the porous substrate fromthermal shrinkage when an electrochemical device overheats. Cavitiespresent between the inorganic particles form fine pores of the porouscoating layer.

The inorganic particles should be present in an amount above apredetermined level in order to allow the porous organic-inorganiccoating layer to inhibit thermal shrinkage of the porous substrate.However, as the content of the inorganic particles increases (that is,as the content of the binder polymer decreases), the bindability of theseparator to an electrode may deteriorate and the inorganic particlesmay be separated from the porous organic-inorganic coating layer whenstress occurs during fabrication (e.g., winding) of an electrochemicaldevice by assembly. The separated inorganic particles act as localdefects of the electrochemical device, giving a negative influence onthe safety of the electrochemical device. Thus, there is a need todevelop a binder polymer that can enhance the bindability of a porousorganic-inorganic coating layer to a porous substrate. There is also aneed to improve the bindability of a porous organic-inorganic coatinglayer to an electrode.

On the other hand, a porous organic-inorganic coating layer having a lowpacking density should be formed to a large thickness so as to performits functions. This acts as an obstacle to the production of a thinseparator essential to enhance the capacity of an electrochemicaldevice.

DISCLOSURE Technical Problem

The present disclosure is designed to solve the problems of the priorart, and therefore it is an object of the present disclosure to providea separator including a porous organic-inorganic coating layer whosepacking density is high enough to realize the fabrication of a thinbattery in an easy manner without losing stability and whose ability tobind to a porous substrate and an electrode is improved, and anelectrochemical device including the separator.

It is another object of the present disclosure to provide a method forproducing the separator in an easy manner.

Technical Solution

According to an aspect of the present disclosure, there is provided aseparator including:

a porous substrate;

a porous organic-inorganic coating layer formed on at least one surfaceof the porous substrate and including a mixture of inorganic particlesand a first binder polymer, the first binder polymer containing acopolymer including (a) a first monomer unit including either at leastone amine group or at least one amide group or both in the side chainthereof and (b) a (meth)acrylate having a C₁-C₁₄ alkyl group as a secondmonomer unit; and

an organic coating layer formed by dispersing a second binder polymer onthe surface of the organic-inorganic coating layer, leaving scattereduncoated areas.

The first monomer unit and the second monomer unit are preferablypresent in amounts of 10 to 80% and 20 to 90% by mole, respectively,based on the total moles of all constituent monomer units of thecopolymer.

As the first monomer unit, there may be used, for example,2-(((butoxyamino)carbonyl)oxy)ethyl (meth)acrylate,2-(diethylamino)ethyl (meth)acrylate, 2-(dimethylamino)ethyl(meth)acrylate, 3-(diethylamino)propyl (meth)acrylate,3-(dimethylamino)propyl (meth)acrylate, methyl2-acetamido(meth)acrylate, 2-(meth)acrylamidoglycolic acid, 2-(meth)acrylamido-2-methyl-1-propanesulfonic acid,(3-(meth)acrylamidopropyl)trimethylammonium chloride,N-(meth)acryloylamido-ethoxyethanol, 3-(meth)acryloylamino-1-propanol,N-(butoxymethyl)(meth)acrylamide, N-tert-butyl(meth)acrylamide,diacetone(meth)acrylamide, N,N-dimethyl(meth)acrylamide,N-(isobutoxymethyl)acrylamide, N-(isopropyl)(meth)acrylamide,(meth)acrylamide, N-phenyl(meth)acrylamide,N-(tris(hydroxymethyl)methyl)(meth)acrylamide,N,N′-(1,3-phenylene)dimaleimide, N,N′-(1,4-phenylene)dimaleimide,N,N′-(1,2-dihydroxyethylene)bisacrylamide,N,N′-ethylenebis(meth)acrylamide or N-vinylpyrrolidinone. These monomerunits may be used alone or as a mixture of two or more thereof. As thesecond monomer unit, there may be used, for example, methyl(meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl(meth)acrylate, n-butyl (meth)acrylate, t-butyl (meth)acrylate,sec-butyl (meth)acrylate, pentyl (meth)acrylate, 2-ethylbutyl(meth)acrylate, 2-ethylhexyl (meth)acrylate, n-octyl (meth)acrylate,isooctyl (meth)acrylate, isononyl (meth)acrylate, lauryl (meth)acrylateor tetradecyl (meth)acrylate. These monomer units may be used alone oras a mixture of two or more thereof.

Preferably, the copolymer further includes (c) a third monomer unitincluding at least one cyano group. The third monomer unit is present inan amount of 5 to 50% by mole, based on the total moles of allconstituent monomer units of the copolymer.

Preferably, the copolymer further includes a monomer unit having atleast one crosslinkable functional group by which the other monomerunits are crosslinked with each other.

The first binder polymer is preferably present in an amount of 2 to 30parts by weight, based on 100 parts by weight of the inorganicparticles. In the porous organic-inorganic coating layer, the firstbinder polymer forms coating layers partially or entirely surroundingthe surfaces of the inorganic particles. The inorganic particles are inclose contact with and are connected and fixed to each other through thecoating layers. Cavities present between the inorganic particles formpores of the porous organic-inorganic coating layer.

The porous organic-inorganic coating layer preferably has a packingdensity D satisfying the inequality: 0.40×D_(inorg)≦D≦0.70×D_(inorg)where D is (Sg−Fg)/(St−Et), Sg is the weight (g) per unit area (m²) ofthe separator in which the porous organic-inorganic coating layer isformed on the porous substrate, Fg is the weight (g) of the unit area(m²) of the porous substrate, St is the thickness (μm) of the separatorin which the porous organic-inorganic coating layer is formed on theporous substrate, Ft is the thickness (μm) of the porous substrate, andD_(inorg) is the density (g/m²×μm) of the inorganic particles used.

The second binder polymer has a solubility parameter different from thatof the first binder polymer. The difference in solubility parameterbetween the first and second binder polymers is preferably at least 4(J/cm³)^(0.5), more preferably at least 8 (J/cm³)^(0.5). As the secondbinder polymer, there may be used, for example, polyvinylidenefluoride-hexafluoropropylene, polyvinylidene fluoride-trichloroethylene,polyacrylonitrile or polyvinylpyrrolidone. These binder polymers may beused alone or as a mixture of two or more thereof.

Preferably, the organic coating layer covers 5 to 80% of the entiresurface area of the organic-inorganic coating layer. The area of theorganic coating layer is more preferably from 10 to 60% of the entiresurface area of the organic-inorganic coating layer.

According to another aspect of the present disclosure, there is provideda method for producing a separator, including:

(S1) preparing a porous substrate;

(S2) dispersing inorganic particles and dissolving a first binderpolymer in a solvent to prepare a slurry, the first binder polymercontaining a copolymer including (a) a first monomer unit includingeither at least one amine group or at least one amide group or both inthe side chain thereof and (b) a (meth)acrylate having a C₁-C₁₄ alkylgroup as a second monomer unit, coating the slurry on at least onesurface of the porous substrate, and drying the slurry to form a porousorganic-inorganic coating layer; and

(S3) coating a solution of 0.2 to 2.0% by weight of a second binderpolymer on the organic-inorganic coating layer, and drying the polymersolution.

According to yet another aspect of the present disclosure, there isprovided an electrochemical device including a cathode, an anode and theseparator interposed between the two electrodes. The electrochemicaldevice may be, for example, a lithium secondary battery or asupercapacitor.

Advantageous Effects

As is apparent from the foregoing, the porous organic-inorganic coatinglayer of the separator according to the present disclosure has a highpacking density and good ability to bind to the porous substrate.Therefore, the separator of the present disclosure has a reducedresistance and can be used to fabricate a thin electrochemical device inan easy manner without losing stability, which contributes to anenhanced capacity of the electrochemical device. In addition, theseparator of the present disclosure is highly resistant to thermal andmechanical impact, which prevents the inorganic particles fromseparating from the porous organic-inorganic coating layer. The organiccoating layer has uncoated areas scattered on the surface of theorganic-inorganic coating layer. This structure can enhance thebindability of the separator to an electrode without a substantialincrease in resistance.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate preferred embodiments of the presentdisclosure and, together with the detailed description of the preferredembodiments, serve to explain the principles of the present disclosure.In the drawings:

FIG. 1 is a cross-sectional view schematically illustrating a separatorof the present disclosure;

FIG. 2 is a SEM image showing the surface of a separator produced inExample 1;

FIG. 3 is a SEM image showing the surface of a separator produced inExample 2;

FIG. 4 is a SEM image showing the surface of a separator produced inExample 3;

FIG. 5 is a SEM image showing the surface of a separator produced inExample 4;

FIG. 6 is a SEM image showing the surface of a separator produced inExample 5;

FIG. 7 is a SEM image showing the surface of a separator produced inExample 6;

FIG. 8 is a SEM image showing the surface of a separator produced inExample 7;

FIG. 9 is a SEM image showing the surface of a separator produced inComparative Example 1;

FIG. 10 is a SEM image showing the surface of a separator produced inComparative Example 2;

FIG. 11 is a SEM image showing the surface of a separator produced inComparative Example 3;

FIG. 12 is a SEM image showing the surface of a separator produced inComparative Example 4;

FIG. 13 is a SEM image showing the surface of a separator produced inComparative Example 5.

BEST MODE

Hereinafter, preferred embodiments of the present disclosure will bedescribed in detail with reference to the accompanying drawings. Priorto the description, it should be understood that the terms used in thespecification and the appended claims should not be construed as limitedto general and dictionary meanings, but interpreted based on themeanings and concepts corresponding to technical aspects of the presentdisclosure on the basis of the principle that the inventor is allowed todefine terms appropriately for the best explanation. Therefore, thedescription proposed herein is just a preferable example for the purposeof illustrations only, not intended to limit the scope of thedisclosure, so it should be understood that other equivalents andmodifications could be made thereto without departing from the spiritand scope of the disclosure at the time of filing the presentapplication.

The present disclosure provides a separator including a poroussubstrate, and a porous organic-inorganic coating layer formed on atleast one surface of the porous substrate and including a mixture ofinorganic particles and a first binder polymer. The first binder polymerused to form the porous organic-inorganic coating layer contains acopolymer including (a) a first monomer unit including either at leastone amine group or at least one amide group or both in the side chainthereof and (b) a (meth)acrylate having a C₁-C₁₄ alkyl group as a secondmonomer unit. The copolymer can be represented by (the first monomerunit)_(m)-(the second monomer unit)_(n) (wherein 0<m<1 and 0<n<1). Thereis no restriction on the arrangement of the first and second monomerunits in the copolymer. For example, the first and second monomer unitsmay be arranged randomly or in blocks.

The first and second monomer units serve to provide a good bindingbetween the inorganic particles or between the inorganic particles andthe porous substrate. The porous organic-inorganic coating layer formedusing the first and second monomer units has few defects and a highpacking density. Therefore, the use of the separator according to thepresent disclosure facilitates the fabrication of a thin battery. Inaddition, the separator of the present disclosure is highly stableagainst external impact and is prevented from separation of theinorganic particles.

As the first monomer unit including either at least one amine group orat least one amide group or both in the side chain thereof, there may beused, for example, 2-(((butoxyamino)carbonyl)oxy)ethyl (meth)acrylate,2-(diethylamino)ethyl (meth)acrylate, 2-(dimethylamino)ethyl(meth)acrylate, 3-(diethylamino)propyl (meth)acrylate,3-(dimethylamino)propyl (meth)acrylate, methyl2-acetamido(meth)acrylate, 2-(meth)acrylamidoglycolic acid, 2-(meth)acrylamido-2-methyl-1-propanesulfonic acid,(3-(meth)acrylamidopropyl)trimethylammonium chloride,N-(meth)acryloylamido-ethoxyethanol, 3-(meth)acryloylamino-1-propanol,N-(butoxymethyl)(meth)acrylamide, N-tert-butyl(meth)acrylamide,diacetone(meth)acrylamide, N,N-dimethyl(meth)acrylamide,N-(isobutoxymethyl)acrylamide, N-(isopropyl)(meth)acrylamide,(meth)acrylamide, N-phenyl(meth)acrylamide,N-(tris(hydroxymethyl)methyl)(meth)acrylamide,N,N′-(1,3-phenylene)dimaleimide, N,N′-(1,4-phenylene)dimaleimide,N,N′-(1,2-dihydroxyethylene)bisacrylamide,N,N′-ethylenebis(meth)acrylamide or N-vinylpyrrolidinone. These monomerunits may be used alone or as a mixture of two or more thereof. Thefirst monomer unit is preferably an acrylic monomer unit.

As the (meth)acrylate having a C₁-C₁₄ alkyl group as a second monomerunit, there may be used, for example, methyl (meth)acrylate, ethyl(meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate,n-butyl (meth)acrylate, t-butyl (meth)acrylate, sec-butyl(meth)acrylate, pentyl (meth)acrylate, 2-ethylbutyl (meth)acrylate,2-ethylhexyl (meth)acrylate, n-octyl (meth)acrylate, isooctyl(meth)acrylate, isononyl (meth)acrylate, lauryl (meth)acrylate ortetradecyl (meth)acrylate. These monomer units may be used alone or as amixture of two or more thereof. If the number of carbon atoms includedin the alkyl group of the second monomer unit is greater than 14, anincrease in non-polarity may appear due to the long alkyl chain, leadingto a reduction in the packing density of the porous organic-inorganiccoating layer.

The first monomer unit is preferably present in an amount of 10 to 80%by mole, more preferably 15 to 80% by mole, based on the total moles ofall constituent monomer units of the copolymer. The presence of thefirst monomer unit in an amount of less than 10% by mole may deterioratethe packing density and binding force of the porous organic-inorganiccoating layer. Meanwhile, the presence of the first monomer unit in anamount of more than 80% by mole may cause an excessive increase in thepacking density of the porous organic-inorganic coating layer, leadingto an excessively high electrical resistance. The second monomer unit ispreferably present in an amount of 20 to 90% by mole, based on the totalmoles of all constituent monomer units of the copolymer. The presence ofthe second monomer unit in an amount of less than 20% by mole maydeteriorate the binding to the porous substrate. Meanwhile, the presenceof the second monomer unit in an amount of more than 90% by mole maydeteriorate the packing properties of the copolymer in the porousorganic-inorganic coating layer due to the relatively low content of thefirst monomer unit.

Preferably, the copolymer further includes (c) a third monomer unitincluding at least one cyano group. As the third monomer unit, there maybe mentioned, for example, ethyl cis-(beta-cyano)(meth)acrylate,(meth)acrylonitrile, 2-(vinyloxy)ethanenitrile,2-(vinyloxy)propanenitrile, cyanomethyl (meth)acrylate, cyanoethyl(meth)acrylate or cyanopropyl (meth)acrylate. The third monomer unit ispreferably present in an amount of 5 to 50% by mole, based on the totalmoles of all constituent monomer units of the copolymer.

Preferably, the copolymer further includes a monomer unit having atleast one crosslinkable functional group by which the other monomerunits are crosslinked with each other. As the crosslinkable functionalgroup, there may be exemplified a hydroxyl group, a primary amine group,a secondary amine group, an acid group, an epoxy group, an oxetanegroup, an imidazole group or an oxazoline group. The copolymer may befurther copolymerized with the monomer having at least one crosslinkablefunctional group, followed by crosslinking. The crosslinking may beperformed by the addition of a curing agent, such as isocyanatecompound, an epoxy compound, an oxetane compound, an aziridine compoundor a metal chelating agent. For example, the monomer having at least onecrosslinkable functional group may be used in an amount of 1 to 20% bymole.

The copolymer may further include one or more other monomer units solong as the objects of the present disclosure are not impaired. Forexample, the copolymer may be further copolymerized with at least one(meth)acrylic acid alkylene oxide adduct to improve the ionicconductivity of the separator. Examples of suitable (meth)acrylic acidalkylene oxide adducts include C₁-C₈ alkoxy diethylene glycol(meth)acrylic acid ester, alkoxy triethylene glycol (meth)acrylic acidester, alkoxy tetraethylene glycol (meth)acrylic acid ester, phenoxydiethylene glycol (meth)acrylic acid ester, alkoxy dipropylene glycol(meth)acrylic acid ester, alkoxy tripropylene glycol (meth)acrylic acidester and phenoxy dipropylene glycol (meth)acrylic acid ester.

It will be obvious to those skilled in the art that the first binderpolymer may be combined with at least one binder polymer other than theabove-mentioned copolymer so long as the objects of the presentdisclosure are not impaired.

The inorganic particles used to form the porous organic-inorganiccoating layer of the separator according to the present disclosure arenot specially limited so long as they are electrochemically stable. Inother words, the inorganic particles can be used without particularlimitation in the present disclosure if they do not undergo oxidationand/or reduction in an operating voltage range applied to anelectrochemical device (for example, 0-5 V for Li/Li⁺). In particular,the use of inorganic particles having the ability to transportions canimprove the conductivity of ions in an electrochemical device, leadingto an improvement in the performance of the electrochemical device.

The use of inorganic particles having a high dielectric constant cancontribute to an increase in the degree of dissociation of anelectrolyte salt, for example, a lithium salt, in a liquid electrolyteto improve the ionic conductivity of the electrolyte solution.

For these reasons, it is preferred that the inorganic particles areselected from inorganic particles having a dielectric constant of atleast 5, preferably at least 10, inorganic particles having the abilityto transport lithium ions, and mixtures thereof. Non-limiting examplesof inorganic particles having a dielectric constant of at least 5include BaTiO₃, Pb(Zr,Ti)O₃ (PZT), Pb_(1-x)La_(x)Zr_(1-y)Ti_(Y)O₃ (PLZT,0<x<1, 0<y<1), Pb(Mg_(1/3)Nb_(2/3))O₃—PbTiO₃ (PMN-PT), hafnia (HfO₂),SrTiO₃, SnO₂, CeO₂, MgO, NiO, CaO, ZnO, ZrO₂, Y₂O₃, Al₂O₃, SiC and TiO₂particles. These inorganic particles may be used alone or as a mixturethereof.

Particularly preferred are BaTiO₃, Pb(Zr,Ti)O₃ (PZT),Pb_(1-x)La_(x)Zr_(1-y)Ti_(Y)O₃ (PLZT, 0<x<1, 0<y<1), Pb(Mg_(1/3)Nb_(2/3)) O₃—PbTiO₃(PMN-PT) and hafnia (HfO₂) whose dielectricconstants are 100 or higher that have piezoelectricity to protect bothelectrodes from internal short circuits when an external impact isapplied, ensuring improved safety of an electrochemical device.Piezoelectricity is a phenomenon in which charges are created as aresult of tension or compression under a certain pressure to generate apotential difference between opposite sides. The use of a mixture of theinorganic particles having a high dielectric constant and the inorganicparticles having the ability to transport lithium ions can produceenhanced synergistic effects.

The inorganic particles having the ability to transport lithium ionsrefer to those that contain lithium atoms and have the function oftransferring lithium ions without storing the lithium. The inorganicparticles having the ability to transport lithium ions contain defectsin their structure through which lithium ions can be transferred andmoved. The presence of the defects can improve the conductivity oflithium ions in a battery, leading to improved battery performance.Non-limiting examples of the inorganic particles having the ability totransport lithium ions include lithium phosphate (Li₃PO₄) particles,lithium titanium phosphate (Li_(x)Ti_(y)(PO₄)_(3,)0<x<2, 0<y<3)particles, lithium aluminum titanium phosphate(Li_(x)Al_(y)Ti_(z)(PO₄)_(3,)0<x<2, 0<y<1, 0<z<3) particles,(LiAlTiP)_(x)O_(y) type glass (0<x<4, 0<y<13) particles such as14Li₂O-9Al₂O₃-38TiO₂-39P₂O₅ particles, lithium lanthanum titanate(Li_(x)La_(y)TiO₃, 0<x<2, 0<y<3) particles, lithium germaniumthiophosphate (Li_(x)Ge_(y)P_(z)S_(w,)0<x<4, 0<y<1, 0<z<1, 0<w<5)particles such as Li_(3.25)Ge_(0.25)P_(0.75)S₄ particles, lithiumnitride (Li_(x)N_(y,)0<x<4, 0<y<2) particles such as Li₃N particles,SiS₂ type glass (Li_(x)Si_(y)S_(z,) 0<x<3, 0<y<2, 0<z<4) particles suchas Li₃PO₄—Li₂S—SiS₂ particles, and P₂S₅ type glass(Li_(x)P_(y)S_(z,)0<x<3, 0<y<3, 0<z<7) particles such as LiI—Li₂S—P₂S₅particles. These inorganic particles may be used alone or as a mixturethereof.

The size of the inorganic particles included in the porousorganic-inorganic coating layer is not limited but is preferably in therange of 0.001 to 10 μm. Within this range, a uniform thickness and anoptimal porosity of the coating layer can be obtained. If the size ofthe inorganic particles is smaller than 0.001 μm, the dispersibility ofthe inorganic particles may deteriorate, which makes it difficult tocontrol the physical properties of the separator. Meanwhile, if the sizeof the inorganic particles is larger than 10 μm, the thickness of theporous organic-inorganic coating layer is increased, which maydeteriorate the mechanical properties of the separator, and the poresize is excessively increased, which may increase the probability thatinternal short circuits will be caused during charging and dischargingof a battery.

The content of the first binder polymer in the porous organic-inorganiccoating layer of the separator according to the present disclosure ispreferably from 2 to 30 parts by weight, more preferably from 5 to 15parts by weight, based on 100 parts by weight of the inorganicparticles. The presence of the first binder polymer in an amount of lessthan 2 parts by weight may cause separation of the inorganic particlesfrom the porous organic-inorganic coating layer. Meanwhile, the presenceof the binder polymer in an amount exceeding 30 parts by weight maycause clogging of the pores of the porous substrate, leading to anincrease in resistance, and may reduce the porosity of the porousorganic-inorganic coating layer.

Preferably, the porous organic-inorganic coating layer formed on theporous substrate has a structure in which the first binder polymer formscoating layers partially or entirely surrounding the surfaces of theinorganic particles, the inorganic particles are in close contact withand are connected and fixed to each other through the coating layers,and cavities present between the inorganic particles form pores. Thatis, the inorganic particles are in close contact with each other andcavities present between the inorganic particles in close contact witheach other become pores of the porous organic-inorganic coating layer.

For high packing density, it is preferred that the size of the cavitiespresent between the inorganic particles is equal to or smaller than theaverage particle diameter of the inorganic particles. The first binderpolymer forming coating layers partially or entirely surrounding thesurfaces of the inorganic particles connects and fixes the inorganicparticles to each other. In addition, the inorganic particles in contactwith the porous substrate are fixed to the porous substrate by the firstbinder polymer.

The packing density D of the porous organic-inorganic coating layer canbe defined as the density of the porous organic-inorganic coating layerloaded at a height of 1 μm from the porous substrate per unit area (m²)of the porous substrate. The packing density D preferably satisfies thefollowing inequality:0.40×D_(inorg)≦D≦0.70×D_(inorg)

where D is (Sg−Fg)/(St−Et), Sg is the weight (g) per unit area (m²) ofthe separator in which the porous organic-inorganic coating layer isformed on the porous substrate, Fg is the weight (g) of the unit area(m²) of the porous substrate, St is the thickness (μm) of the separatorin which the porous organic-inorganic coating layer is formed on theporous substrate, Ft is the thickness (μm) of the porous substrate, andD_(inorg) is the density (g/m²×μm) of the inorganic particles used. Whentwo or more kinds of inorganic particles are used, D_(inorg) isdetermined taking into consideration the densities and fractions of theindividual kinds of inorganic particles.

If the packing density D is below the lower limit, the porousorganic-inorganic coating layer becomes structurally loose, posing arisk that the porous organic-inorganic coating layer may lose itsfunction to suppress thermal shrinkage of the porous substrate and itsresistance to mechanical impact may also deteriorate. Meanwhile, if thepacking density D is above the upper limit, the increased packingdensity may bring about an improvement in the physical properties of theporous organic-inorganic coating layer but the decreased porosity of theporous organic-inorganic coating layer may deteriorate the electricalconductivity of the separator.

The thickness of the porous organic-inorganic coating layer composed ofthe inorganic particles and the first binder polymer is not specificallylimited but is preferably in the range of 0.5 to 10 μm.

The porous substrate may be any of those that are commonly used inelectrochemical devices. For example, the porous substrate may be madeof at least one polymer selected from the group consisting ofpolyolefin, polyethylene terephthalate, polybutylene terephthalate,polyacetal, polyamide, polycarbonate, polyimide, polyether ether ketone,polyethersulfone, polyphenylene oxide, polyphenylene sulfide andpolyethylene naphthalene. The porous substrate may be in the form of amembrane or a non-woven fabric. The thickness of the porous substrate ispreferably from 5 to 50 μm but is not particularly limited to thisrange. The pore size and porosity of the porous substrate are preferablyfrom 0.001 to 50 μm and from 10 to 95%, respectively, but are notparticularly limited to these ranges.

The separator includes an organic coating layer formed by dispersing asecond binder polymer on the surface of the organic-inorganic coatinglayer, leaving scattered uncoated areas. The organic coating layer formsthe outer surface of the separator but does not completely cover theentire surface of the organic-inorganic coating layer. The uncoatedareas, in which the second binder polymer is not coated, are scatteredon the surface of the organic-inorganic coating layer. That is, theuncoated areas and the coated areas are dispersed on the surface of theorganic-inorganic coating layer. Ions can pass through the uncoatedareas scattered on the surface of the organic-inorganic coating layer.Due to this structure, the bindability of the separator to an electrodecan be enhanced without a substantial increase in resistance.

Since the second binder polymer constituting the organic coating layeris different from the first binder polymer constituting theorganic-inorganic coating layer, the two binder polymers have differentsolubility parameters. The first and second binder polymers may becopolymers having the same kinds of monomers. Even in this case, sincethe first and second binder polymers include different amounts of themonomers, they have different solubility parameters. The first andsecond binder polymers may be composed of mixtures of the same two kindsof polymers. Even in this case, since the first and second binderpolymers include different amounts of the polymers, they have differentsolubility parameters.

The difference in solubility parameter between the first and secondbinder polymers is preferably at least 4 (J/cm³)^(0.5), more preferablyat least 8 (J/cm³)^(0.5). Examples of polymers suitable for use as thesecond binder polymer include, but are not limited to, polyvinylidenefluoride-hexafluoropropylene, polyvinylidene fluoride-trichloroethylene,polyacrylonitrile and polyvinylpyrrolidone. These polymers may be usedalone or as a mixture of two or more thereof.

Taking both the bindability of the separator to an electrode and theresistance of the separator into account, it is preferred that theorganic coating layer covers 5 to 80% of the entire surface area of theorganic-inorganic coating layer. The area of the organic coating layeris more preferably from 10 to 60% of the entire surface area of theorganic-inorganic coating layer. The thickness of the organic coatinglayer is preferably from 0.1 to 2 μm, more preferably from 0.1 to 1 μm.

FIG. 1 is a cross-sectional view schematically illustrating theseparator of the present disclosure, which includes the elementsdescribed above. In the separator 10 illustrated in FIG. 1, the porousorganic-inorganic coating layer is formed on the porous substrate 1 andincludes the inorganic particles 3 and the first binder polymer 5. Theorganic coating layer 7 is formed by dispersing the second binderpolymer on the surface of the organic-inorganic coating layer, leavingscattered uncoated areas.

The present disclosure also provides a method for producing theseparator. The method is preferably carried out by the followingprocedure.

First, a porous substrate is prepared (S1). The kind of the poroussubstrate is as described above.

Subsequently, a copolymer including a first monomer unit and a secondmonomer unit is prepared and is dissolved alone or in combination withone or more other binder polymers in a solvent to prepare a solution ofa first binder polymer. Inorganic particles are added to and dispersedin the first binder polymer solution. The solvent preferably has asolubility parameter similar to that of the first binder polymer and hasa low boiling point, which are advantageous for uniform mixing and easeof solvent removal. Non-limiting examples of solvents usable to dissolvethe copolymer include acetone, tetrahydrofuran, methylene chloride,chloroform, dimethylformamide, N-methyl-2-pyrrolidone (NMP),cyclohexane, and water. These solvents may be used alone or as a mixturethereof. It is preferred to crush the inorganic particles after additionto the first binder polymer solution. At this time, the crushing issuitably performed for 1 to 20 hr. The inorganic particles arepreferably crushed to a particle size of 0.001 to 10 μm. The inorganicparticles may be crushed by any suitable technique known in the art.Ball milling is particularly preferred.

The first binder polymer solution containing the inorganic particlesdispersed therein is coated on the porous substrate and dried to form aporous organic-inorganic coating layer (S2). The coating is preferablyperformed at a humidity of 10 to 80%. Any drying process for evaporatingthe solvent may be employed, such as hot-air drying.

The binder polymer solution containing the inorganic particles dispersedtherein may be coated on the porous substrate by a suitable techniqueknown in the art, for example, dip coating, die coating, roll coating,comma coating or a combination thereof. The porous organic-inorganiccoating layer may be formed on either one or both surfaces of the poroussubstrate.

Subsequently, 0.2 to 2.0% by weight of a second binder polymer isdissolved in a solvent, coated on the organic-inorganic coating layer,and dried (S3).

The second binder polymer solution may be coated by any of theabove-mentioned techniques for coating the first binder polymersolution. The second binder polymer solution coated on the entiresurface of the organic-inorganic coating layer is self-assembled on thesurface of the organic-inorganic coating layer during solventevaporation to form an organic coating layer. As described above, theorganic coating layer forms the outer surface of the separator but doesnot completely cover the entire surface of the organic-inorganic coatinglayer. As a result of the self-assembly, uncoated areas are scatteredbetween the coated areas on the organic-inorganic coating layer.Different solubility parameters of the first and second binder polymersare required to obtain the shape of the organic coating layer. Inaddition, it is necessary to control the concentration of the secondbinder polymer solution.

If the second binder polymer solution has the same composition as thefirst binder polymer solution, it has a high affinity for theorganic-inorganic coating layer after coating and drying. Accordingly, alarge amount of the second binder polymer solution permeates the poresof the organic-inorganic coating layer, leaving only a slight amount ofthe second binder polymer on the surface of the organic-inorganiccoating layer. If the content of the second binder polymer in thesolution is less than 0.2% by weight, the effect of improving thebinding to an electrode may be negligible. Meanwhile, if the content ofthe second binder polymer in the solution exceeds 2.0% by weight, it isdifficult to obtain the desired shape of the organic coating layerhaving uncoated areas. It should, of course, be understood that theconcentration of the second binder polymer solution may vary dependingon the kind of the second binder polymer.

In view of the foregoing, it is preferred to limit the content of thesecond binder polymer in the solution to the range defined above, toselect the second binder polymer from those described above and toadjust the thickness of the organic coating layer to the range definedabove.

Although combinations of the elements described as preferred embodimentshave not been particularly disclosed in the specification, it should beunderstood that combinations of two or more of the elements can beadopted as various constitutions of the present disclosure.

The present disclosure also provides an electrochemical device using theseparator. Specifically, the electrochemical device of the presentdisclosure has a structure in which the separator is interposed betweenand laminated to a cathode and an anode. The electrochemical deviceincludes all devices in which electrochemical reactions occur. Specificexamples of such electrochemical devices include all kinds of primarybatteries, secondary batteries, fuel cells, solar cells, and capacitorssuch as supercapacitors. Particularly preferred are lithium secondarybatteries, including lithium metal secondary batteries, lithium ionsecondary batteries, lithium polymer secondary batteries and lithium ionpolymer secondary batteries.

The electrochemical device can be fabricated by suitable methods knownin the art. As an example, the electrochemical device may be fabricatedby interposing the separator between a cathode and an anode, assemblingthe electrode structure, and injecting an electrolyte solution into theelectrode assembly.

There is no particular restriction on the production method of thecathode and the anode to be applied together with the separator of thepresent disclosure. Each of the electrodes can be produced by binding anelectrode active material to an electrode current collector by suitablemethods known in the art. The cathode active material may be any ofthose that are commonly used in cathodes of conventional electrochemicaldevices. Non-limiting examples of particularly preferred cathode activematerials include lithiated manganese oxides, lithiated cobalt oxides,lithiated nickel oxides, lithiated iron oxides, and composite oxidesthereof. The anode active material may be any of those that are commonlyused in anodes of conventional electrochemical devices. Non-limitingexamples of particularly preferred anode active materials includelithium, lithium alloys, and lithium intercalation materials, such ascarbon, petroleum coke, activated carbon, graphite and other carbonmaterials. Non-limiting examples of cathode current collectors suitablefor use in the cathode include aluminum foils, nickel foils, andcombinations thereof. Non-limiting examples of anode current collectorssuitable for use in the anode include copper foils, gold foils, nickelfoils, copper alloy foils, and combinations thereof.

The electrochemical device of the present disclosure can use anelectrolyte solution consisting of a salt and an organic solvent capableof dissolving or dissociating the salt. The salt has a structurerepresented by A⁺B⁻ wherein A⁺ is an alkali metal cation, such as Li⁺,Na⁺, K⁺ or a combination thereof, and B⁻ is an anion, such as PF₆ ⁻, BF₄⁻, Cl⁻, Br⁻, I⁻, ClO₄ ⁻, AsF₆ ⁻, CH₃CO₂ ⁻, CF₃SO₃ ⁻, N(CF₃SO₂)₂ ⁻,C(CF₂SO₂)₃ ⁻ or a combination thereof. Examples of organic solventssuitable for dissolving or dissociating the salt include, but are notlimited to, propylene carbonate (PC), ethylene carbonate (EC), diethylcarbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC),dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane,tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethyl methyl carbonate(EMC) and γ-butyrolactone. These organic solvents may be used alone oras a mixture thereof.

The electrolyte solution may be injected in any suitable step duringfabrication of the battery depending on the manufacturing processes anddesired physical properties of a final product. Specifically, theelectrolyte solution may be injected before battery assembly or in thefinal step of battery assembly.

Hereinafter, embodiments of the present disclosure will be described indetail. The embodiments of the present disclosure, however, may takeseveral other forms, and the scope of the present disclosure should notbe construed as being limited to the following examples. The embodimentsof the present disclosure are provided to more fully explain the presentdisclosure to those having ordinary knowledge in the art to which thepresent disclosure pertains.

Preparation of Copolymers

Copolymers having the monomer compositions shown in Table 1 wereprepared.

TABLE 1 Monomer Copolymer 1 Copolymer 2 Copolymer 3 DMAAm 40 31 — DMAEA20 4 35 AN 40 15 15 EA — 46 30 BA — — 28 AA — 4 — HBA — — 2

In Table 1, the abbreviations DMAAm, DMAEA, AN, EA, BA, AA and HBA meanN,N-dimethylacrylamide, N,N-dimethylaminoethyl acrylate, acrylonitrile,ethyl acrylate, n-butyl acrylate and hydroxybutyl acrylate,respectively.

Examples and Comparative Examples

Separators having the compositions shown in Table 2 were produced by thefollowing procedure.

First, the corresponding copolymer and an epoxy curing agent weredissolved in acetone to prepare a solution of a first binder polymer. Tothe binder polymer solution were added alumina particles in such anamount that the binder polymer, the curing agent and the inorganicparticles were in a weight ratio of 7.15:0.35:92.5. The inorganicparticles were crushed to a particle diameter of about 400 nm anddispersed by ball milling for at least 3 hr to prepare a slurry.

The slurry was dip coated on both surfaces of a 12 μm thick porouspolyethylene film (porosity=45%), followed by drying to formorganic-inorganic coating layers.

Subsequently, PVdF-HFP polymers having different HFP contents weredissolved in acetone to have the concentrations shown in Table 2. Eachof the solutions was dip coated on the porous film on which theorganic-inorganic coating layers had been formed, followed by drying toform organic coating layers.

The separator was cut into a specimen having a size of 50 mm×50 mm. Theair permeability of the specimen and the packing density D of theorganic-inorganic coating layer were measured. The results are shown inTable 3.

The air permeability was evaluated as the time taken for 100 ml air tocompletely pass through the specimen.

The thermal shrinkage of the specimen in the machine direction wasmeasured after storage at 150° C. for 1 hr.

After the specimen was fixed to a glass plate by using a double-sidedtape, a transparent tape (3M) was fixedly attached to the exposed porousorganic-inorganic coating layer. The force (gf/15 mm) needed for peelingthe tape was measured using a tensile tester and was defined as theadhesive strength of the separator.

TABLE 2 Organic-inorganic coating layer Organic coating layer ThicknessThickness Total of porous after HFP content Concentration thickness filmbefore formation Packing of PVdF- of polymer of coating of coatingCopolymer density HFP solution separator (μm) layer (μm) used (D)copolymer (wt %) (μm) Example 1 12 ± 0.5 16 ± 0.5 Copolymer 1 0.58 ×D_(inorg) 6 mole % 0.5 16.0 ± 0.5 Example 2 12 ± 0.5 16 ± 0.5 Copolymer2 0.55 × D_(inorg) 6 mole % 1.0 16.0 ± 0.5 Example 3 12 ± 0.5 16 ± 0.5Copolymer 3 0.57 × D_(inorg) 6 mole % 2.0 17.0 ± 0.5 Example 4 12 ± 0.516 ± 0.5 Copolymer 2 0.55 × D_(inorg) 15 mole % 0.5 16.0 ± 0.5 Example 512 ± 0.5 16 ± 0.5 Copolymer 2 0.55 × D_(inorg) 15 mole % 1.0 16.0 ± 0.5Example 6 12 ± 0.5 16 ± 0.5 Copolymer 2 0.55 × D_(inorg) 15 mole % 1.516.0 ± 0.5 Example 7 12 ± 0.5 16 ± 0.5 Copolymer 2 0.55 × D_(inorg) 15mole % 2.0 16.3 ± 0.5 Comparative 12 ± 0.5 16 ± 0.5 Copolymer 2 0.55 ×D_(inorg) — — 16.0 ± 0.5 Example 1 Comparative 12 ± 0.5 16 ± 0.5Copolymer 1 0.58 × D_(inorg) 6 mole % 0.1 16.0 ± 0.5 Example 2Comparative 12 ± 0.5 16 ± 0.5 Copolymer 2 0.55 × D_(inorg) 6 mole % 2.518.0 ± 0.5 Example 3 Comparative 12 ± 0.5 16 ± 0.5 Copolymer 2 0.55 ×D_(inorg) 15 mole % 0.1 16.0 ± 0.5 Example 4 Comparative 12 ± 0.5 16 ±0.5 Copolymer 2 0.55 × D_(inorg) 15 mole % 2.5 17.0 ± 0.5 Example 5Comparative 12 ± 0.5 16 ± 0.5 Copolymer 2 0.55 × D_(inorg) Copolymer 21.0 16.5 ± 0.5 Example 6 Comparative 12 ± 0.5 16 ± 0.5 PVdF-HFP 0.38 ×D_(inorg) 6 mole % 1.0 16.5 ± 0.5 Example 7 (6 mole %)

TABLE 3 Physical properties of separators Electrical resistance AdhesiveTotal Air of coin strength Lamination MD thermal thickness permeabilitycell (gf/15 force shrinkage (μm) (s/100 ml) (ohm) mm) (gf/15 mm) (%)Example 1 16.0 ± 0.5 342-362 0.92-1.05 33-52 15-22 <8 Example 2 16.0 ±0.5 346-353 0.93-1.07 32-48 19-25 <8 Example 3 17.0 ± 0.5 344-4590.92-1.05 32-45 23-26 <8 Example 4 16.0 ± 0.5 345-356 0.93-1.04 30-5215-19 <8 Example 5 16.0 ± 0.5 343-367 0.92-1.09 32-44 18-23 <8 Example 616.0 ± 0.5 351-382 0.95-1.10 33-42 21-28 <8 Example 7 16.3 ± 0.5 372-4030.98-1.10 33-45 24-29 <6 Comparative 16.0 ± 0.5 341-370 0.93-1.04 35-500 <8 Example 1 Comparative 16.0 ± 0.5 338-373 0.95-1.03 32-49 0-2 <8Example 2 Comparative 18.0 ± 0.5 362-380 0.98-1.13 26-43 20-24 <8Example 3 Comparative 16.0 ± 0.5 335-368 0.90-1.02 31-54 0 <8 Example 4Comparative 17.0 ± 0.5 503-630 1.18-1.32 33-47 21-26 <8 Example 5Comparative 16.5 ± 0.5 >1000 >1.41 24-43 18-25 <8 Example 6 Comparative16.5 ± 0.5 450-650 1.15-1.30 1-4 1-4  50> Example 7

Production of Anodes

96 wt % of carbon powder as an anode active material, 3 wt % ofpolyvinylidene fluoride (PVdF) as a binder and 1 wt % of carbon black asa conductive material were added to N-methyl-2-pyrrolidone (NMP) as asolvent to prepare a slurry. The slurry was applied to a 10 μm thickcopper (Cu) foil as an anode collector and dried to produce an anode,which was then roll pressed.

Production of Cathodes

92 wt % of a lithiated cobalt composite oxide as a cathode activematerial, 4 wt % of carbon black as a conductive material and 4 wt % ofPVDF as a binder were added to N-methyl-2-pyrrolidone (NMP) as a solventto prepare a slurry. The slurry was applied to a 20 μm thick aluminum(Al) foil as a cathode collector and dried to produce a cathode, whichwas then roll pressed.

Fabrication of Batteries

The anode, the cathode and the separator were stacked to construct anelectrode assembly. An electrolyte consisting of ethylene carbonate(EC)/ethyl methyl carbonate (EMC) (1:2, v/v) and 1 mole of lithiumhexafluorophosphate (LiPF₆) was injected into the electrode assembly tofabricate a battery.

The cycle performance of the battery was tested at room temperature and60° C. The results are shown in Table 4. Surface images of theseparators produced in Examples 1-7 and Comparative Examples 1-5 weretaken by scanning electron microscopy (SEM) and are shown in FIGS. 2-13,respectively.

TABLE 4 Room temperature (cycle) 60° C. (cycle) 100 200 300 600 100 200300 600 Example 1 97- 93- 89- 84- 97- 92- 86- 81- 99% 95% 93% 87% 99%95% 91% 85% Example 2 97- 95- 92- 86- 97- 95- 87- 82- 99% 97% 94% 88%99% 96% 93% 85% Example 3 96- 94- 91- 86- 97- 92- 87- 83- 98% 96% 93%87% 99% 95% 91% 84% Example 4 97- 94- 91- 85- 97- 93- 85- 80- 99% 96%93% 87% 99% 96% 90% 84% Example 5 97- 95- 91- 86- 97- 93- 88- 83- 99%96% 93% 88% 99% 96% 91% 86% Example 6 97- 94- 90- 85- 97- 92- 89- 83-99% 96% 92% 87% 99% 96% 91% 85% Example 7 96- 94- 90- 83- 97- 92- 89-81- 98% 96% 92% 85% 99% 96% 91% 84% Comparative — — — — — — — — Example1 Comparative — — — — — — — — Example 2 Comparative 91- 87- 84- 75- 93-86- 79- 68- Example 3 93% 90% 86% 80% 95% 90% 84% 75% Comparative — — —— — — — — Example 4 Comparative 80- 72- 64- <65% 83- 75- 62- <60%Example 5 85% 78% 68% 89% 80% 68% Comparative 72- 62- 60- <60% 78- 72-65- <65% Example 6 78% 68% 65% 88% 82% 72% Comparative — — — — — — — —Example 7

In Table 4, “-” means that it was impossible to fabricate a batterybecause the binding force between the electrode and the separator waseither weak or nonexistent.

What is claimed is:
 1. A method for producing a separator, comprising:(S1) preparing a porous substrate; (S2) dispersing inorganic particlesand dissolving a first binder polymer in a solvent to prepare a slurry,the first binder polymer containing a copolymer comprising (a) a firstmonomer unit comprising either at least one amine group or at least oneamide group or both in the side chain thereof and (b) a (meth)acrylatehaving a C₁-C₁₄ alkyl group as a second monomer unit, coating the slurryon at least one surface of the porous substrate, and drying the slurryto form a porous organic-inorganic coating layer; and (S3) coating asolution of 0.2 to 2.0% by weight of a second binder polymer on theorganic-inorganic coating layer, and drying the polymer solution.
 2. Themethod according to claim 1, wherein the second binder polymer has asolubility parameter different by at least 4 (J/cm³)^(0.5) from that ofthe first binder polymer.
 3. The method according to claim 1, whereinthe second binder polymer is selected from the group consisting ofpolyvinylidene fluoride-hexafluoropropylene, polyvinylidenefluoride-trichloroethylene, polyacrylonitrile, polyvinylpyrrolidone, andmixtures thereof.